1. Field of the Invention
The present invention relates to the field of color video signal processing circuits and more particularly to a contour accentuation circuit for a color signal for use in video equipment such as a video cassette recorder or a television receiver.
2. Description of the Related Art
In a video cassette recorder or a television receiver, contour accentuation circuits are used for enhancement of clarity of pictures. In this case, contour accentuation means to steepen the rising edges or falling edges of wave forms. Typically, in general, a contour accentuation circuit includes a variable gain amplifier for amplifying an input signal, a detector for detecting the output signal from the amplifier and a differential circuit for differentiating the output signal from the detector and controlling the gain of the variable gain amplifier in accordance with the output control signal of the differential circuit.
In this prior art contour accentuation circuit, if the variable gain amplifier is constructed as an integrated circuit (IC), the amplifier is typically a Gilbert cell circuit which generally has a control characteristic related to the emitter equivalent resistance of a transistor. This emitter equivalent resistance r is nearly proportional to the emitter current of the transistor; therefore, the control characteristic of the amplifier is proportional to the gain control voltage of the amplifier. If a rising edge portion of a color signal is contour-accentuated, the larger the input signal of the amplifier becomes, the larger is the resultant gain of the amplifier, because the control voltage output of the differential circuit becomes larger (the differentiating value of the rising edge of the input wave form has a trend of being larger).
If the characteristic of the variable gain amplifier is determined to optimize the contour-accentuation for a color signal of large amplitude wave form, then accentuating the contour for a color signal of small amplitude becomes inadequate. Meanwhile, if the characteristic of the variable gain amplifier is determined to optimize the contour-accentuation for a color signal of small amplitude wave form, then the contour accentuating for the color signal of large amplitude becomes over-accentuating. In this over-accentuating case, a problematic ringing occurs at the rising edge portion of the color signal whose contours are accentuated (see lines 500 shown in FIG. 5).